If Frederick Jackson Turner had anything right in "The Frontier Thesis", although he didn't pay hardly any attention to the South, this idea of a safety valve of a West to move to was surely there for slavery.

如果弗雷德里克·杰克逊·特纳,在《边境论》中有什么正确的言论,那就是,尽管他对南方几乎没有施以任何关注,即这个西进运动是一道安全阀的构想,显然是为奴隶制而设的

耶鲁公开课 - 美国内战与重建课程节选